Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow to make or keep them economically viable. The servicing of oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently ladened with corrosive acids and abrasive propants such as sharp sand.
In order to protect the components which make up the wellhead, such as the valves, tubing hanger, casing hanger, casing head and the blowout preventer equipment, wellhead isolation tools are used during fracturing and stimulating procedures. The wellhead isolation tools generally insert a mandrel through the various valves and spools of the wellhead to isolate those components from the elevated pressures and from the corrosive and abrasive fluids used in the well treatment to stimulate production. One example of those well isolation tools is described in the Applicant's U.S. patent application Ser. No. 09/537,629, entitled BLOWOUT PREVENTER PROTECTOR AND METHOD OF USING SAME filed Mar. 29, 2000. Another example of such a tool is described in the Applicant's U.S. Pat. No. 4,867,243 which issued Sep. 19, 1989 and is entitled WELLHEAD ISOLATION TOOL AND SETTING TOOL AND METHOD OF USING SAME. In those examples a top end of the mandrel is connected to one or more high pressure valves through which the stimulation fluids are pumped. A pack-off assembly is provided at a bottom end of the mandrel for achieving a fluid seal against an inside of the production tubing or well casing, so that the wellhead is completely isolated from the stimulation fluids.
Various pack-off assemblies provided at a bottom end of the mandrel of wellhead isolation tools are described in other prior art patents, such as U.S. Pat. No. 4,023,814, entitled A TREE SAVER PACKER CUP, which issued to Pitts on May 17, 1977; U.S. Pat. No. 4,111,261, entitled A WELLHEAD ISOLATION TOOL, which issued to Oliver on Sep. 5, 1978; U.S. Pat. No. 4,601,494, entitled A NIPPLE INSERT, which issued to McLeod et al. on Jul. 22, 1986, and Canadian Patent 1,272,684, entitled A WELLHEAD ISOLATION TOOL NIPPLE, which issued to Sutherland-Wenger on Aug. 14, 1990. These pack-off assemblies include a cup tool that radially expands under high fluid pressures to seal against the inside wall of a production tubing or casing.
In an effort to improve existing pack-off assemblies and to further improve the high pressure seal, McLeod et al. in U.S. Pat. No. 5,261,487, entitled PACKOFF NIPPLE, which issued on Nov. 16, 1993, describe a packoff nipple for use on a mandrel of a wellhead isolation tool. This tool is described below with reference to FIGS. 1—3.
FIG. 1 shows McLeod et al's sealing nipple assembly 100, which is attached to the wellhead isolation tool mandrel 98, in a non-actuated condition. The sealing nipple assembly 100 includes a cylindrical nipple body 104, which slidably receives thereon an elastomeric primary seal 106 having a forward lip (more clearly shown in FIG. 2) and an elastomeric packer ring 108. The elastomeric primary seal 106 and the elastomeric packer ring 108 are bonded to respective rigid seal rings 110 and 112 such that the elastomeric primary seal 106 and the elastomeric packer ring 108 are axially movable relative to the cylindrical nipple body 104. O-rings 114 and 116 are provided between the cylindrical nipple body 104 and the respective rigid seal rings 110 and 112. The axial movements of the elastomeric primary seal 106 and the elastomeric packer ring 108 are restrained between a shoulder 118 of the cylindrical nipple body 104 and a shoulder sub 120. The cylindrical nipple body has a bottom end that terminates in a bullnose 122 for guiding the pack-off nipple assembly 100 into the tubing 96. The shoulder sub 120 which is threadedly connected to the top end of the cylindrical collar 104 has a lower end having two angular shoulders 126 and 128.
Under elevated fluid pressures 132, as shown in FIGS. 2 and 3, the elastomeric primary seal 106 expands radially to establish a primary seal between the pack-off nipple assembly 100 and the tubing 96 such that the elastomeric primary seal 106 is forced upwardly to move the elastomeric packer ring 108 upwardly against angled first and second shoulders 126, 128 of the shoulder sub 120. A sealing shoulder 130 of the elastomeric packer ring 108 is forced upwardly under high fluid pressures 132, over the first angled shoulder 126, and extrudes into an annular gap between the tubing 96 and the external periphery of the lower angular shoulder 126. This is shown in FIG. 2. When the fluid pressures 132 are further elevated, the elastomeric packer ring 108 is forced further upward and the sealing shoulder 130 further intrudes into an annular gap between the tubing 96 and the second angled shoulder 128, as shown in FIG. 3. Thus, the elastomeric pack-off nipple assembly 100 provides a seal between the mandrel 98 and the tubing 96 in order to inhibit fluid leakage under very high fluid pressures, until the mandrel 98 is withdrawn from the tubing 96, which causes the sealing shoulder 130 of the elastomeric packer ring 108 to slide off the angular shoulders 126 and 128.
The elastomers used for the primary seal 106 and the packer ring 108 are of different hardness. The packer ring 108 is preferably made of an elastomer having a greater durometer than that of the primary seal 106. Thus, the harder packer ring 108 is able to withstand greater wear, while the softer primary seal 106 is able to flex when the nipple assembly 100 is inserted into the tubing 96. Preferred durometer values are 80 for the primary seal 106 and 95 for the elastomeric packer ring 108.
Although McLeod et al's pack-off nipple assembly is reported to provide an adequate seal, the assembly has at least one disadvantage. During insertion of the mandrel, the elastomeric packer ring 108 may be prematurely actuated to extrude into the annular gap between the tubing 96 and the respective angled shoulders 126, 128. This can occur when the primary seal 106 and the elastomeric packer ring 108 are forced through a constriction in a wellhead during the insertion of the mandrel 98 into the tubing 96. The frictional forces acting on the primary seal 106 and the elastomeric packer ring 108 can cause the elastomeric packer ring 108 to be frictionally trapped while the pack-off nipple assembly 100 is moving downwardly with the mandrel 98. The angled first and second shoulders 126 and 128 readily permit the sealing shoulder 130 of the elastomeric packer ring 108 to move upwardly. Once this occurs, the further insertion of the mandrel 98 can tear the elastomeric packing ring 108, which may result in a malfunction of the tool.
There is therefore, a need for further improvements in pack-off assemblies for use with a mandrel of wellhead isolation tools.